Hologram medium

ABSTRACT

The present disclosure relates to a hologram recording medium having one surface with a higher surface energy than a polymer resin layer containing at least one polymer selected from the group consisting of triacetyl cellulose, alicyclic olefin polymer and polyethylene terephthalate, a hologram recording medium wherein the surface energy of any one surface is 50 mN/m or more, and an optical element comprising the hologram medium.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a 35 U.S.C. 371 National Phase Entry Applicationfrom PCT/KR2019/011874, filed on Sep. 11, 2019, designating the UnitedStates, which claims the benefit of Korean Patent Application No.10-2018-0115322 filed on Sep. 27, 2018 with the Korean IntellectualProperty Office, the disclosures of which are herein incorporated byreference in their entirety.

TECHNICAL FIELD

The present disclosure relates to a hologram medium and an opticalelement.

BACKGROUND OF THE INVENTION

Hologram recording medium records information by changing a refractiveindex in the holographic recording layer in the medium through anexposure process, reads the variation of refractive index in the mediumthus recorded, and reproduces the information.

When a photopolymer (photosensitive resin) is used, the lightinterference pattern can be easily stored as a hologram byphotopolymerization of the low molecular weight monomer. Therefore, thephotopolymer can be used in various fields such as optical lenses,mirrors, deflecting mirrors, filters, diffusing screens, diffractionelements, light guides, waveguides, holographic optical elements havingprojection screen and/or mask functions, medium of optical memory systemand light diffusion plate, optical wavelength multiplexers, reflectiontype, transmission type color filters, and the like.

Typically, a photopolymer composition for hologram production comprisesa polymer binder, a monomer, and a photoinitiator, and thephotosensitive film produced from such a composition is irradiated withlaser interference light to induce photopolymerization of localmonomers.

In a portion where a relatively large number of monomers are present insuch photopolymerization process, the refractive index becomes high. Andin a portion where a relatively large number of polymer binders arepresent, the refractive index is relatively lowered and thus therefractive index modulation occurs, and a diffraction grating isgenerated by such refractive index modulation. The value n of refractiveindex modulation is influenced by the thickness and the diffractionefficiency (DE) of the photopolymer layer, and the angular selectivityincreases as the thickness decreases.

Recently, development of materials capable of maintaining a stablehologram with a high diffraction efficiency has been demanded, and alsovarious attempts have been made to manufacture a photopolymer layerhaving a large value of refractive index modulation even while having athin thickness.

BRIEF SUMMARY OF THE INVENTION

The present disclosure provides a hologram medium which can preventlight loss due to reflection, scattering, and absorption caused by theinfluence of a light-transmissive substrate, can realize a higherrefractive index modulation value and diffraction efficiency even in therange of a thin thickness, and can provide lighter and smaller-sizedproducts by removing the substrate when applied to VR (Virtual Reality)devices or AR (Augmented Reality) devices.

The present disclosure also provides an optical element comprising theabove-mentioned hologram medium.

Provided herein is a hologram recording medium having one surface with ahigher surface energy than a polymer resin layer containing at least onepolymer selected from the group consisting of triacetyl cellulose,alicyclic olefin polymer and polyethylene terephthalate.

Also provided herein is a hologram recording medium in which the surfaceenergy of any one surface is 50 mN/m or more.

Further provided herein is an optical element including theabove-mentioned hologram medium.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a hologram medium and an optical element according tospecific embodiments of the present disclosure will be described in moredetail.

As used herein, the term “(meth)acrylate” refers to either methacrylateor acrylate.

As used herein, the term “(co)polymer” refers to either a homopolymer ora copolymer (including a random copolymer, a block copolymer, and agraft copolymer).

As used herein, the term “hologram” refers to a recording medium inwhich optical information is recorded in an entire visible range and anear ultraviolet range (300 to 800 nm) through an exposure process, andexamples thereof include all of visual holograms such as in-line (Gabor)holograms, off-axis holograms, full-aperture transfer holograms, whitelight transmission holograms (“rainbow holograms”), Denisyuk holograms,off-axis reflection holograms, edge-lit holograms or holographicstereograms.

In the present specification, the alkyl group may be a straight chain ora branched chain, and the number of carbon atoms thereof is notparticularly limited, but is preferably 1 to 40. According to oneembodiment, the alkyl group has 1 to 20 carbon atoms. According toanother embodiment, the alkyl group has 1 to 10 carbon atoms. Accordingto still another embodiment, the alkyl group has 1 to 6 carbon atoms.Specific examples of the alkyl group include methyl, ethyl, propyl,n-propyl, isopropyl, butyl, n-butyl, isobutyl, tert-butyl, sec-butyl,1-methyl-butyl, 1-ethyl-butyl, pentyl, n-pentyl, isopentyl, neopentyl,tert-pentyl, hexyl, n-hexyl, 1-methylpentyl, 2-methylpentyl,4-methyl-2-pentyl, 3,3-dimethylbutyl, 2-ethylbutyl, heptyl, n-heptyl,1-methylhexyl, cyclopentylmethyl, cyclohexylmethyl, octyl, n-octyl,tert-octyl, 1-methylheptyl, 2-ethylhexyl, 2-propylpentyl, n-nonyl,2,2-dimethylheptyl, 1-ethyl-propyl, 1,1-dimethyl-propyl, isohexyl,2-methylpentyl, 4-methylhexyl, 5-methylhexyl, and the like, but are notlimited thereto.

In the present specification, the alkylene group is a bivalentfunctional group derived from alkane, and may be, for example, linear,branched or cyclic methylene group, ethylene group, propylene group,isobutylene group, sec-butylene group, tert-butylene group, pentylenegroup, hexylene group, and the like.

According to one embodiment of the present disclosure, there may beprovided a hologram recording medium having one surface with a highersurface energy than a polymer resin layer containing at least onepolymer selected from the group consisting of triacetyl cellulose,alicyclic olefin polymer and polyethylene terephthalate.

According to another embodiment of the present disclosure, there may beprovided a hologram recording medium in which a surface energy of anyone surface is 50 mN/m or more.

The present inventors have prepared a hologram recording medium having ahigher surface energy than a polymer resin layer containing at least onepolymer selected from the group consisting of triacetyl cellulose,alicyclic olefin polymer and polyethylene terephthalate, or having asurface energy of any one surface of 50 mN/m or more, and have foundthrough experiments that such a hologram recording medium can be easilypeeled off from a polymer resin layer or other substrate containing atleast one polymer selected from the group consisting of triacetylcellulose, alicyclic olefin polymer and polyethylene terephthalatewithout separate surface damage or deterioration of physical properties.

Usually, the hologram recording medium is used in the form of beingcoated on an optical substrate such as triacetyl cellulose, alicyclicolefin polymer and polyethylene terephthalate, but light loss due toreflection, scattering, and absorption occurs due to the opticalsubstrate. In addition, previously known hologram recording media orphotopolymers for producing the same have not been easily peeled offafter coating on the above-mentioned optical substrate, and there was alimit that surface properties or physical properties at the time ofpeeling are deteriorated.

On the contrary, as the hologram recording medium of one embodiment hashigher surface energy than the polymer resin layer containing at leastone polymer selected from the group consisting of triacetyl cellulose,alicyclic olefin polymer and polyethylene terephthalate, it can beeasily peeled off from a commonly used optical substrate or the like,without deteriorating surface properties or physical properties, andthus, it can be applied to various fields while having more improvedoptical properties.

Further, the hologram recording medium of the other embodiment has asurface energy of any one surface of 50 mN/m or more, so that it can beeasily peeled off from a commonly used optical substrate withoutdeterioration of surface properties or physical properties, and thus canbe applied to various fields while having more improved opticalproperties.

The hologram recording medium of the one embodiment may have one surfacewith a higher surface energy than a polymer resin layer containing atleast one polymer selected from the group consisting of triacetylcellulose, alicyclic olefin polymer and polyethylene terephthalate, andthe specific surface energy of this one surface may be 50 mN/m or more,or 50 mN/m to 60 mN/m.

In addition, the hologram recording medium according to anotherembodiment may have a surface energy of any one surface of 50 mN/m ormore, or 50 to 60 mN/m.

The surface characteristics possessed by the hologram recording mediumof the above-described embodiments are considered to result from thecharacteristics of the polymer substrate contained therein.

More specifically, the hologram recording medium may include a polymersubstrate including a polymer resin in which a silane-based functionalgroup is located in a main chain or a branched chain.

Since the polymer resin in which the silane-based functional group islocated in the main chain or branched chain has a high equivalent weightof silane as the main functional group and has a very low sol-gelcrosslinking degree, it appears that the surface energy is relativelyhigh. Further, alkyl chains in the form of acrylate and polyethyleneoxide with relatively high surface tension occupy most of the coatinglayer, and the silicone component or fluorine compound of the matrix canimprove the release property of the coating layer.

One surface of the polymer substrate may have a higher surface energythan a polymer resin layer containing at least one polymer selected fromthe group consisting of triacetyl cellulose, alicyclic olefin polymerand polyethylene terephthalate, and one surface of the polymer substratemay have a surface energy of 50 mN/m or more, or 50 mN/m to 60 mN/m.

Meanwhile, a fine pattern may be formed on the other one surface of thehologram recording media of the above-described embodiments, that is, ona surface opposite to the one surface having the above-mentioned surfaceenergy characteristics.

In the polymer substrate wherein the silane-based functional group isformed from a photopolymer composition containing a polymer resinlocated in a main chain or a branched chain, in the hologram recordingprocess using light such as a coherent laser or electromagneticradiation, diffusion of photoreactive monomers is uniformly generatedand thus, curing shrinkage of the polymer substrate is also uniformlygenerated.

Thereby, according to the curing shrinkage of the polymer substrate thatis uniformly generated, a fine pattern may be uniformly and regularlyformed on at least one surface of the polymer substrate (on a surfaceopposite to one surface having the above-mentioned surface energy), andsuch hologram media can achieve high diffraction efficiency.

In addition, the hologram medium of the embodiments comprises a polymersubstrate including a polymer resin in which the silane-based functionalgroup is located in a main chain or a branched chain, thereby realizingsignificantly improved refractive index modulation value and excellentresistance to temperature and humidity compared to previously knownholograms.

The fine pattern may have a shape including two or more fine protrusionsin which a maximum value of the height and a minimum value of the heightare alternately repeated based on the cross section of the polymersubstrate.

The standard error of the distance in the cross-sectional direction ofthe polymer substrate between the maximum value of the height of the onefine protrusion and the minimum value of the height of the adjacent fineprotrusion may be 20 nm or less, or 1 to 20 nm.

As the standard error of the distance in the cross-sectional directionof the polymer substrate between the maximum value of the height of theone fine protrusion and the minimum value of the height of the adjacentfine protrusion is 20 nm or less, the hologram medium can have moreimage sharpness or higher diffraction efficiency.

On the contrary, when the standard error of the distance in thecross-sectional direction of the polymer substrate between the maximumvalue of the height of the one fine protrusion and the minimum value ofthe height of the adjacent fine protrusion is higher than the aboverange, image distortion phenomenon may occur from the hologram medium,additional scattering or interference of diffracted light may occur, andthe diffraction efficiency may also be reduced.

The standard error of the distance in the cross-sectional direction ofthe polymer substrate between the maximum value of the height of the onefine protrusion and the minimum value of the height of the adjacent fineprotrusion may be defined by the following general formula 1.

SE=σ/√{square root over (n)}  [General Formula 1]

in the general formula 1, SE is a standard error, σ is a standarddeviation, and n is a number.

Further, the standard error of the distance in the cross-sectionaldirection of the polymer substrate between the maximum value of theheight of the one fine projection and the minimum value of the adjacentfine projection can be determined as a standard deviation of the samplemean distribution between adjacent minimum and maximum values in theline profile of the surface shape of the hologram medium measured by anatomic force microscope.

Meanwhile, the distance in the cross-sectional direction of the polymersubstrate between the maximum value of the height of the one fineprojection and the minimum value of the adjacent fine projection may be0.2 μm to 2 μm, or 0.3 μm to 1 μm.

Further, the amplitude which is the difference between the maximum valueof the height of the one fine projection and the minimum value of theheight of the adjacent fine projection may be 5 to 50 nm.

As described above, the polymer substrate may include a polymer resin inwhich silane-based functional group is located in a main chain or abranched chain. Examples of the polymer resin are not particularlylimited, and specific examples thereof include (meth)acrylate-based(co)polymers in which a silane-based functional group is located in amain chain or a branched chain.

More specifically, the polymer substrate may include a cross-linkedproduct between a polymer matrix including a (meth)acrylate-based(co)polymer in which a silane-based functional group is located in abranched chain and a silane crosslinking agent; and a photoreactivemonomer.

A crosslinked structure mediating a siloxane bond can be easilyintroduced through a sol-gel reaction between a modified(meth)acrylate-based (co)polymer containing a silane-based functionalgroup and a silane crosslinking agent containing a terminal silane-basedfunctional group, and excellent durability against temperature andhumidity can be ensured through such siloxane bond. Further, asdescribed above, in the hologram recording process, diffusion ofphotoreactive monomers and curing shrinkage of the polymer substrate canbe uniformly generated, and thereby, the curing shrinkage of the polymersubstrate is also uniformly generated. Due to the curing shrinkage ofthe polymer substrate that is uniformly generated, a fine pattern may beuniformly and regularly formed on at least one surface of the polymersubstrate. Thus, the hologram media of the embodiment can achieve highdiffraction efficiency.

In the polymer matrix, the (meth)acrylate-based (co)polymer in which thesilane-based functional group is located in the branched chain and thesilane crosslinking agent can be present as separate components,respectively, and they may also exist in the form of a complex formed byreacting with each other.

In the (meth)acrylate-based (co)polymer, the silane-based functionalgroup may be located in a branched chain. The silane-based functionalgroup may include a silane functional group or an alkoxysilanefunctional group. Preferably, a trimethoxysilane group can be used asthe alkoxysilane functional group.

The silane-based functional group may form siloxane bonds through asol-gel reaction with the silane-based functional group contained in thesilane crosslinking agent to crosslink the (meth)acrylate-based(co)polymer and the silane crosslinking agent.

Meanwhile, the (meth)acrylate-based (co)polymer in which thesilane-based functional group is located in the branched chain mayinclude a (meth)acrylate repeating unit in which the silane-basedfunctional group is located in a branched chain, and a (meth)acrylaterepeating unit.

An example of the (meth)acrylate repeating unit in which thesilane-based functional group is located in a branched chain may includea repeating unit represented by the following Chemical Formula 1.

in the Chemical Formula 1, R₁ to R₃ are each independently alkyl grouphaving 1 to 10 carbon atoms, R₄ is hydrogen or an alkyl group having 1to 10 carbon atoms, and R₅ is an alkylene group having 1 to 10 carbonatoms.

Preferably, in Chemical Formula 1, R₁ to R₃ are each independently amethyl group having one carbon atom, R₄ is a methyl group having onecarbon atom and R₅ is a propylene group having 3 carbon atoms, which maybe a repeating unit derived from methacryloxypropyltrimethoxysilane(KBM-503), or R₁ to R₃ are each independently a methyl group having onecarbon atom, R₄ is hydrogen and R₅ is a propylene group having 3 carbonatoms, which may be a repeating unit derived from3-acryloxypropyltrimethoxysilane (KBM-5103).

Further, an example of the (meth)acrylate repeating unit may include arepeating unit represented by the following Chemical Formula 2.

in the Chemical Formula 2, R₆ is an alkyl group having 1 to 20 carbonatoms, R₇ is hydrogen or an alkyl group having 1 to 10 carbon atoms.Preferably, in the Chemical Formula 2, R₆ is a butyl group having 4carbon atoms and R₇ is hydrogen, which may be a repeating unit derivedfrom butyl acrylate.

The molar ratio between the repeating unit of Chemical Formula 2 and therepeating unit of Chemical Formula 1 may be 0.5:1 to 14:1. When themolar ratio of the repeating unit of Chemical Formula 1 is excessivelyreduced, the crosslinking density of the matrix becomes too low to serveas a support, resulting in a decrease in recording characteristics afterrecording. When the molar ratio of the repeating unit of ChemicalFormula 1 is excessively increased, the crosslinking density of thematrix becomes too high and the mobility of the respective componentsdecreases, resulting in a decrease in the refractive index modulationvalue.

The weight average molecular weight (measured by GPC) of the(meth)acrylate-based (co)polymer may be 100,000 to 5,000,000, or 300,000to 900,000. The weight average molecular weight means a weight averagemolecular weight (unit: g/mol) converted in terms of polystyrenemeasured by GPC method. In the process of determining the weight averagemolecular weight converted in terms of polystyrene measured by the GPCmethod, a commonly known analyzing device, a detector such as arefractive index detector, and an analytical column can be used.Commonly applied conditions for temperature, solvent, and flow rate canbe used. Specific examples of the measurement conditions man include atemperature of 30° C., chloroform solvent and a flow rate of 1 mL/min.

Meanwhile, in the (meth)acrylate-based (co)polymer in which thesilane-based functional group is located in a branched chain, theequivalent weight of the silane-based functional group may be 300 g/eq.to 2000 g/eq., or 500 g/eq. to 2000 g/eq., or 550 g/eq. to 1800 g/eq.,or 580 g/eq. to 1600 g/eq., or 586 g/eq. to 1562 g/eq. The silane-basedfunctional group equivalent weight is an equivalent weight (g/eq.) ofone silane-based functional group, and is a value obtained by dividingthe weight average molecular weight of the (meth)acrylate-based(co)polymer by the number of silane-based functional groups permolecule. The smaller the equivalent value, the higher the density ofthe functional group, and the larger the equivalent value, the smallerthe density of the functional group.

Consequently, the crosslinking density between the (meth)acrylate-based(co)polymer and the silane crosslinking agent can be optimized, therebyensuring superior durability against temperature and humidity ascompared with conventional matrixes. In addition, through theoptimization of the crosslinking density as described above, themobility between the photoreactive monomer having a high refractiveindex and a component having a low refractive index is increased,thereby maximizing the modulation of the refractive index and improvingrecording characteristics.

When the equivalent weight of the silane-based functional groupcontained in the (meth)acrylate-based (co)polymer in which thesilane-based functional group is located in a branched chain isexcessively reduced to less than 300 g/eq., the crosslinking density ofthe matrix becomes too high and the mobility of components is inhibited,thereby causing a decrease in the recording characteristics.

Further, when the equivalent weight of the silane-based functional groupcontained in the (meth)acrylate-based (co)polymer in which thesilane-based functional group is located in a branched chain isexcessively increased to more than 2000 g/eq., the crosslinking densityis too low to serve as a support, so that the boundary surface betweenthe diffraction gratings generated after recording collapses, and thevalue of the refractive index modulation may decrease over time.

Meanwhile, the silane crosslinking agent may be a compound having anaverage of at least one silane-based functional group per molecule or amixture thereof, and it may be a compound containing at least onesilane-based functional group. The silane-based functional group mayinclude a silane functional group or an alkoxysilane functional group.Preferably, a triethoxysilane group can be used as the alkoxysilanefunctional group. The silane-based functional group can form siloxanebonds through a sol-gel reaction with the silane-based functional groupcontained in the (meth)acrylate-based (co)polymer, thereby crosslinkingthe (meth)acrylate-based (co)polymer and the silane crosslinking agent.

In this case, the silane crosslinking agent may have a silane-basedfunctional group equivalent weight of 200 g/eq. to 1000 g/eq. Thus, thecrosslinking density between the (meth)acrylate-based (co)polymer andthe silane crosslinking agent can be optimized, thereby ensuringsuperior durability against temperature and humidity as compared withconventional matrixes. In addition, through the optimization of thecrosslinking density as described above, the mobility between aphotoreactive monomer having a high refractive index and a componenthaving a low refractive index can be increased, thereby maximizing therefractive index modulation and improving the recording characteristics.

When the equivalent weight of the silane-based functional groupcontained in the silane crosslinking agent is excessively increased tomore than 1000 e/eq., the diffraction grating interface after recordingmay be collapsed due to the decrease of the crosslinking density of thematrix. In addition, due to the loose crosslinking density and the lowglass transition temperature, the monomer and plasticizer components canbe eluted on the surface to cause haze. When the equivalent weight ofthe silane-based functional group contained in the silane crosslinkingagent is excessively reduced to less than 200 g/eq., the crosslinkingdensity becomes too high and the mobility of the monomer and theplasticizer components is inhibited, thereby causing a problem that therecording characteristics are remarkably lowered.

More specifically, the silane crosslinking agent may include a linearpolyether main chain having a weight average molecular weight of 100 to2000, or 300 to 1000, or 300 to 700 and a silane functional group bondedto the main chain terminal or the branched chain.

The linear polyether main chain having a weight average molecular weightof 100 to 2000 may include a repeating unit represented by the followingChemical Formula 3.

—(R₈O)_(n)—R₈—  [Chemical Formula 3]

in the Chemical Formula 3, R₈ is an alkylene group having 1 to 10 carbonatoms, and n is an integer of 1 or more, or 1 to 50, or 5 to 20, or 8 to10.

As the linear silane crosslinking agent introduces flexible polyetherpolyol into a main chain, the mobility of the components can be improvedthrough adjustment of the glass transition temperature and crosslinkingdensity of the matrix.

Meanwhile, a bond between the silane-based functional group and thepolyether main chain may be mediated by a urethane bond. Specifically,the silane-based functional group and the polyether main chain may forma bond between them via a urethane bond. More specifically, the siliconatom contained in the silane-based functional group binds directly tothe nitrogen atom of the urethane bond or via an alkylene group having 1to 10 carbon atoms. The functional group R₈ contained in the polyethermain chain can be directly bonded to the oxygen atom of the urethanebond.

The silane-based functional group and the polyether main chain beingbonded via a urethane bond in this way is because the silanecrosslinking agent is a reaction product produced through the reactionbetween an isocyanate compound containing a silane-based functionalgroup and a linear polyether polyol compound having a weight averagemolecular weight of 100 to 2000.

More specifically, the isocyanate compound may include an aliphatic,cycloaliphatic, aromatic or aromatic-aliphatic mono-isocyanate,di-isocyanate, tri-isocyanate or poly-isocyanate; or oligo-isocyanate orpoly-isocyanate of diisocyanate or triisocyante having urethane, urea,carbodiimide, acylurea, isocyanurate, allophanate, biuret,oxadiazinetrione, uretdione or iminooxadiazinedione structures.

Specific examples of the isocyanate compound containing the silane-basedfunctional group include 3-isocyanatopropyltriethoxysilane.

Further, the polyether polyol may be, for example, multiple additionproducts of styrene oxide, ethylene oxide, propylene oxide,tetrahydrofuran, butylene oxide, and epichlorohydrin, and their mixedaddition products and graft products, polyether polyols and polyhydricalcohols obtained by condensation of polyhydric alcohols or mixturesthereof, and those obtained by alkoxylation of amines and aminoalcohols.

Specific examples of the polyether polyol include polypropylene oxide),polyethylene oxide) and combinations thereof, or poly(tetrahydrofuran)and mixtures thereof, which are in the form of random or blockcopolymers having an OH functionality of 1.5 to 6 and a number averagemolecular weight between 200 and 18000 g/mol, preferably an OHfunctionality of 1.8 to 4.0 and a number average molecular weightbetween 600 and 8000 g/mol, particularly preferably an OH functionalityof 1.9 to 3.1 and a number average molecular weight between 650 to 4500g/mol.

In this way, when the silane-based functional group and the polyethermain chain are bonded via a urethane bond, a silane crosslinking agentcan be more easily synthesized.

The silane crosslinking agent has a weight average molecular weight(measured by GPC) of 1000 to 5,000,000. The weight average molecularweight means a weight average molecular weight converted in terms ofpolystyrene determined by GPC method. In the process of determining theweight average molecular weight in terms of polystyrene measured by theGPC method, a commonly known analyzing device, a detector such as arefractive index detector, and an analytical column can be used.Commonly applied conditions for temperature, solvent, and flow rate canbe used. Specific examples of the measurement conditions may include atemperature of 30° C., chloroform solvent, and a flow rate of 1 mL/min.

Meanwhile, the content of the silane crosslinking agent may be 10 partsby weight to 90 parts by weight, or 20 parts by weight to 70 parts byweight, or 22 parts by weight to 65 parts by weight based on 100 partsby weight of the (meth)acrylate-based (co)polymer.

When the content of the silane crosslinking agent in the reactionproduct is excessively reduced relative to 100 parts by weight of the(meth)acrylate-based (co)polymer, the curing rate of the matrix isremarkably slow and so the function as a support is lost, and thediffraction grating interface after recording can be easily collapsed.When the content of the silane crosslinking agent in the reactionproduct is excessively increased relative to 100 parts by weight of the(meth)acrylate-based (co)polymer, the curing rate of the matrix becomesfaster, but compatibility issues with other components arises due to anexcessive increase in the content of the reactive silane group and thushaze occurs.

In addition, the modulus (storage modulus) of the reaction product maybe 0.01 MPa to 5 MPa. As a specific example of the modulus measuringmethod, the value of storage modulus (G′) can be measured at a frequencyof 1 Hz at room temperature (20° C. to 25° C.) using a DHR (DiscoveryHybrid Rheometer) equipment from TA Instruments.

Further, the glass transition temperature of the reaction product may be−40° C. to 10° C. A specific example of the glass transition temperaturemeasuring method includes a method of measuring a change in phase angle(loss modulus) of the film coated with a photopolymerizable compositionin the temperature range from −80° C. to 30° C. under the settingconditions of strain 0.1%, frequency 1 Hz, temperature raising rate 5°C./min by using a DMA (dynamic mechanical analysis) measuring equipment.

Meanwhile, the photoreactive monomer may include a polyfunctional(meth)acrylate monomer or a monofunctional (meth)acrylate monomer.

As described above, in a portion where the monomer is polymerized in theprocess of photopolymerization of the photopolymer composition and thepolymer is present in relatively large amounts, the refractive indexbecomes high. In a portion where the polymer binder is present inrelatively large amounts, the refractive index becomes relatively low,the refractive index modulation occurs, and a diffraction grating isgenerated by such refractive index modulation.

Specifically, one example of the photoreactive monomer may include(meth)acrylate-based (α,β-unsaturated carboxylic acid derivatives, forexample, (meth)acrylate, (meth)acrylamide, (meth)acrylonitrile,(meth)acrylic acid or the like, or a compound containing a vinyl groupor a thiol group.

One example of the photoreactive monomer may include a polyfunctional(meth)acrylate monomer having a refractive index of 1.5 or more, or 1.53or more, or 1.5 to 1.7. The polyfunctional (meth)acrylate monomer havinga refractive index of 1.5 or more, or 1.53 or more, or 1.5 to 1.7 mayinclude a halogen atom (bromine, iodine, etc.), sulfur (S), phosphorus(P), or an aromatic ring.

More specific examples of the polyfunctional (meth)acrylate monomerhaving the refractive index of 1.5 or more include bisphenol A modifieddiacrylate type, fluorene acrylate type (HR6022, Miwon SpecialtyChemical Co., Ltd.), bisphenol fluorene epoxy acrylate type (HR6100,HR6060, HR6042, etc.—Miwon Specialty Chemical Co., Ltd.), halogenatedepoxy acrylate type (HR1139, HR3362., etc.—Miwon Specialty Chemical Co.,Ltd.).

Another example of the photoreactive monomer may include amonofunctional (meth)acrylate monomer. The monofunctional (meth)acrylatemonomer may contain an ether bond and a fluorene functional group in themolecule. Specific examples of such monofunctional (meth)acrylatemonomer include phenoxybenzyl (meth)acrylate, o-phenylphenol ethyleneoxide (meth)acrylate, benzyl (meth)acrylate, 2-(phenylthio)ethyl(meth)acrylate, biphenylmethyl (meth)acrylate, or the like.

On the other hand, the photoreactive monomer may have a weight averagemolecular weight of 50 g/mol to 1000 g/mol, or 200 g/mol to 600 g/mol.The weight average molecular weight means a weight average molecularweight in terms of polystyrene measured by GPC method.

The photopolymer composition may include 1% to 80% by weight of thepolymer matrix or a precursor thereof; 1% to 80% by weight of thephotoreactive monomer; and 0.1% to 20% by weight of the photoinitiator.When the photopolymer composition further includes an organic solvent asdescribed below, the content of the above-mentioned components is basedon the total sum of these components (the total sum of the componentsexcluding the organic solvent).

The hologram medium of the embodiments can realize a refractive indexmodulation value (n) of 0.020 or more or 0.021 or more, or 0.02:2 ormore or 0.023 or more, or 0.020 to 0.035, or 0.027 to 0.030 even at athickness of 5 μm to 30 μm.

Further, the hologram medium can realize a diffraction efficiency of 50%or more, or 85% or more, 85 to 99% at a thickness of 5 μm to 30 μm.

In the photopolymer composition forming the hologram medium, therespective components contained therein are uniformly mixed, dried andcured at a temperature of 20° C. or higher, and then predeterminedexposure procedures were undertaken, thereby producing a hologram foroptical application in the entire visible range and the near ultravioletregion (300 to 800 nm).

In the photopolymer composition, the components of forming a polymermatrix or the precursor thereof are first uniformly mixed. Subsequently,the above-described silane crosslinking agent is mixed with the catalystto prepare a hologram-forming process.

In the photopolymer composition, a mixing device, a stirrer, a mixer, orthe like which are commonly known in the art can be used for mixing therespective components contained therein without particular limitation.The temperature in the mixing process can be 0° C. to 100° C.,preferably 10° C. to 80° C., particularly preferably 20° C. to 60° C.

Meanwhile, the components of forming the polymer matrix or the precursorthereof in the photopolymer composition are first homogenized and mixed.Subsequently, at the time of adding the silane crosslinking agent, thephotopolymer composition can be a liquid formulation that is cured at atemperature of 20° C. or more.

The curing temperature may vary depending on the composition of thephotopolymer and the curing is promoted, for example, by heating at atemperature of from 30° C. to 180° C.

At the time of curing, the photopolymer may be in state of beinginjected into or coated onto a predetermined substrate or mold.

Meanwhile, as the method of recording a visual hologram on a hologrammedium produced from the photopolymer composition, generally knownmethods can be used without particular limitation. The method describedin the holographic recording method described later can be adopted asone example.

The photopolymer composition may further include a fluorine-basedcompound.

Since the fluorine-based compound has stability with little reactivityand has low refractive index characteristics, the refractive index ofthe polymer matrix can be further lowered and thus the refractive indexmodulation with the monomer can be maximized.

The fluorine-based compound may include at least one functional groupselected from the group consisting of an ether group, an ester group andan amide group, and at least two difluoromethylene groups. Morespecifically, the fluorine-based compound may have a structure of thefollowing Chemical Formula 4 in which a functional group containing anether group is bonded to both terminals of a central functional groupcontaining a direct bond or an ether bond between two difluoromethylenegroups.

in the Chemical Formula 4, R₁₁ and R₁₂ are each independently adifluoromethwaylene group, R₁₃ and R₁₆ are each independently amethylene group, R₁₄ and R₁₅ are each independently a difluoromethylenegroup, R₁₇ and R₁₈ are each independently a polyalkylene oxide group,and m is an integer of 1 or more, or 1 to 10, or 1 to 3.

Preferably, in Chemical Formula 4, R₁₁ and R₁₂ are each independently adifluoromethylene group, R₁₃ and R₁₆ are each independently a methylenegroup, R₁₄ and R₁₅ are each independently a difluoromethylene group, R₁₇and R₁₈ are each independently 2-methoxyethoxymethoxy group, and m is aninteger of 2.

The fluorine-based compound may have a refractive index of less than1.45, or 1.3 or more and less than 1.45. As described above, since thephotoreactive monomer has a refractive index of 1.5 or more, thefluorine-based compound can further lower the refractive index of thepolymer matrix through a refractive index lower than that of thephotoreactive monomer, thereby maximizing the refractive indexmodulation with the monomer.

Specifically, the content of the fluorine-based compound may be 30 partsby weight to 150 parts by weight, or 50 parts by weight to 110 parts byweight based on 100 parts by weight of the photoreactive monomer, andthe refractive index of the polymer matrix may be 1.46 to 1.53.

When the content of the fluorine-based compound is excessively reducedrelative to 100 parts by weight of the photoreactive monomer, the valueof the refractive index modulation after recording is lowered due to thedeficiency of low refractive index components. When the content of thefluorine-based compound is excessively increased relative to 100 partsby weight of the photoreactive monomer, haze occurs due to compatibilityissues with other components, or some fluorine-based compounds may beeluted on the surface of the coating layer.

The fluorine-based compound may have a weight average molecular weight(measured by GPC) of 300 or more, or 300 to 1000. A specific method ofmeasuring the weight average molecular weight is as described above.

Meanwhile, the polymer substrate may further include a photosensitizingdye.

The photosensitizing dye serves as photosensitizing pigment to sensitizethe photoinitiator. More specifically, the photosensitizing dye may bestimulated by the light irradiated on the photopolymer composition andmay also serve as an initiator to initiate polymerization of the monomerand the crosslinking monomer. The photopolymer composition forming thepolymer substrate may contain 0.01% by weight to 30% by weight, or 0.05%by weight to 20% by weight of the photosensitizing dye.

Examples of the photosensitizing dye are not particularly limited, andvarious compounds commonly known in the art can be used. Specificexamples of the photosensitizing dye include sulfonium derivative ofceramidonine, new methylene blue, thioerythrosine triethylammonium,6-acetylamino-2-methylceramidonin, eosin, erythrosine, rose bengal,thionine, basic yellow, Pinacyanol chloride, Rhodamine 6G, Gallocyanine,ethyl violet, Victoria blue R, Celestine blue), QuinaldineRed, CrystalViolet, Brilliant Green, Astrazon orange G, Darrow Red, Pyronin Y, BasicRed 29, pyrylium iodide, Safranin O, Cyanine, Methylene Blue, Azure A,or a combination of two or more thereof.

The photopolymer composition forming the hologram medium of theembodiments can be provided as a hologram medium after uniformly mixingthe respective components contained therein, followed by drying andcuring at a temperature of 20° C. or higher. Therefore, the photopolymercomposition may further include an organic solvent for mixing and easierapplication of each component included therein.

Non-limiting examples of the organic solvent include ketones, alcohols,acetates and ethers, or mixtures of two or more thereof. Specificexamples of such organic solvent include ketones such as methyl ethylketone, methyl isobutyl ketone, acetylacetone or isobutyl ketone;alcohols such as methanol, ethanol, n-propanol, i-propanol, n-butanol,i-butanol or t-butanol; acetates such as ethyl acetate, i-propylacetate, or polyethylene glycol monomethyl ether acetate; ethers such astetrahydrofuran or propylene glycol monomethyl ether; or a mixture oftwo or more thereof.

The organic solvent may be added at the time of mixing the respectivecomponents contained in the photopolymer composition, or may becontained in the photopolymer composition while adding the respectivecomponents in a state of being dispersed or mixed in an organic solvent.When the content of the organic solvent in the photopolymer compositionis too low, the flowability of the photopolymer composition may belowered, resulting in the occurrence of defects such as the occurrenceof striped patterns on the finally produced film. In addition, when theorganic solvent is added excessively, the solid content becomes low,coating and film formation are not sufficiently performed, and thus, thephysical properties and surface characteristics of the film may bedeteriorated and defects may occur during the drying and curing process.Thus, the photopolymer composition may include an organic solvent suchthat the total solid content concentration of the components containedis 1% by weight to 70% by weight, or 2% by weight to 50% by weight.

Meanwhile, the photopolymer composition forming the polymer substratemay include a photoinitiator. The photoinitiator is a compound which isactivated by light or actinic radiation and initiates polymerization ofa compound containing a photoreactive functional group such as thephotoreactive monomer.

As the photoinitiator, commonly known photoinitiators can be usedwithout particular limitation, but specific examples thereof include aphotoradical polymerization initiator, a photocationic polymerizationinitiator, and a photoanionic polymerization initiator.

Specific examples of the photoradical polymerization initiator includeimidazole derivatives, bisimidazole derivatives, N-aryl glycinederivatives, organic azide compounds, titanocene, aluminate complex,organic peroxide, N-alkoxypyridinium salts, thioxanthone derivatives,amine derivative or the like. More specifically, examples of thephotoradical polymerization initiator include3-di(t-butyldioxycarbonyl)benzophenone,3,3′,4,4″-tetrakis(t-butyldioxycarbonyl)benzophenone,3-phenyl-5-isoxazolone, 2-mercapto benzimidazole,bis(2,4,5-triphenyl)imidazole, 2,2-dimethoxy-1,2-diphenylethane-1-one(product name: Irgacure 651/manufacturer: BASF),1-hydroxy-cyclohexyl-phenyl-ketone (product name: Irgacure184/manufacturer: BASF),2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1 (productname: Irgacure 369/manufacturer: BASF), andbis(η5-2,4-cyclopentadiene-1-yl)-bis(2,6-difluoro-3-(1H-pyrrole-1-yl)-phenyl)titanium(product name: Irgacure 784/manufacturer: BASF), EbecrylP-115(manufacturer: SK entis), or the like.

The photo cationic polymerization initiator may include a diazoniumsalt, a sulfonium salt, or an iodonium salt, and examples thereofinclude sulfonic acid esters, imidosulfonates,dialkyl-4-hydroxysulfonium salts, arylsulfonic acid-p-nitrobenzyl ester,silanol-aluminum complexes, (η6-benzene) (η5-cyclopentadienyl)iron (II),or the like. In addition, benzoin tosylate, 2,5-dinitrobenzyltosylate,N-tosylphthalic acid imide, or the like can be mentioned. More specificexamples of the photocationic polymerization initiator includecommercially available products such as Cyracure UVI-6970, CyracureUVI-6974 and Cyracure UVI-6990 (manufacturer: Dow Chemical Co. in USA),Irgacure 264 and Irgacure 250 (manufacturer: BASF) or CIT-1682(manufacturer: Nippon Soda).

The photo anionic polymerization initiator may include a borate salt,and examples thereof include butyrylchlorine butyl triphenylborate orthe like. More specific examples of the photoanionic polymerizationinitiator include commercially available products such as Borate V(manufacturer: Spectra Group) or the like.

In addition, the photopolymer composition may include monomolecular(type I) initiator or bimolecular (type II) initiator. The (type I)system for free radical photopolymerization may include, for example, anaromatic ketone compounds in combination with a tertiary amine, such asbenzophenone, alkylbenzophenone, 4,4′-bis(dimethylamino)benzophenone(Michler's ketone), anthrone and halogenated benzophenone or a mixtureof these types. The bimolecular (type II) initiator may include benzoinand derivatives thereof, benzyl ketal, acylphosphine oxide, for example,2,4,6-trimethylbenzoyldiphenylphosphine oxide, bisacylophosphine oxide,phenylglyoxyl ester, camphorquinone, alpha-aminoalkylphenone,alpha-dialkoxyacetophenone, 1-[4-(phenylthio)phenyl]octane-1,2-dione2-(O-benzoyloxime), alpha-hydroxyalkylphenone, and the like.

The photopolymer composition may further include other additives,catalysts, and the like. For example, the photopolymer composition mayinclude a catalyst which is commonly known for promoting polymerizationof the polymer matrix or photoreactive monomer.

Examples of the catalyst include tin octanoate, zinc octanoate,dibutyltin dilaurate, dimethylbis[(1-oxoneodecyl)oxy]stannane,dimethyltin dicarboxylate, zirconium bis(ethylhexanoate), zirconiumacetylacetonate, p-toluenesulfonic acid or tertiary amines such as1,4-diazabicyclo[2.2.2]octane, diazabicyclononane, diazabicycloundecane, 1,1,3,3-tetramethylguanidine,1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido(1,2-a)pyrimidine, and thelike.

Examples of the other additives include a defoaming agent or aphosphate-based plasticizer. As the defoaming agent, a silicone-basedreactive additive may be used, and an example thereof is Tego Rad 2500.Examples of the plasticizer include phosphate compounds such as tributylphosphate. The plasticizer may be added together with the fluorine-basedcompound at a weight ratio of 1:5 to 5:1. The plasticizer may have arefractive index of less than 1.5 and a molecular weight of 700 or less.

Meanwhile, the hologram medium can be provided by a recording method ofa hologram medium comprising selectively polymerizing photoreactivemonomers contained in the photopolymer composition by a coherent laser.

As described above, through the process of mixing and curing thephotopolymer composition, it is possible to produce a medium in a formin which no visual hologram is recorded, and a visual hologram can berecorded on the medium through a predetermined exposure process.

A visual hologram can be recorded on the media provided through theprocess of mixing and curing the photopolymer composition, using knowndevices and methods under commonly known conditions.

Meanwhile, according to another embodiment of the present disclosure, anoptical element including a hologram medium can be provided.

Specific examples of the optical element include optical lenses,mirrors, deflecting mirrors, filters, diffusing screens, diffractionelements, light guides, waveguides, holographic optical elements havingprojection screen and/or mask functions, medium of optical memory systemand light diffusion plate, optical wavelength multiplexers, reflectiontype, transmission type color filters, and the like.

An example of an optical element including the hologram medium mayinclude a hologram display device.

The hologram display device includes a light source unit, an input unit,an optical system, and a display unit. The light source unit is aportion that irradiates a laser beam used for providing, recording, andreproducing three-dimensional image information of an object in theinput unit and the display unit. Further, the input unit is a portionthat previously inputs three-dimensional image information of an objectto be recorded on the display unit, and for example, three-dimensionalinformation of an object such as the intensity and phase of light foreach space can be inputted into an electrically addressed liquid crystalSLM, wherein an input beam may be used. The optical system may include amirror, a polarizer, a beam splitter, a beam shutter, a lens, and thelike. The optical system can be distributed into an input beam forsending a laser beam emitted from the light source unit to the inputunit, a recording beam for sending the laser beam to the display unit, areference beam, an erasing beam, a reading beam, and the like.

The display unit can receive three-dimensional image information of anobject from an input unit, record it on a hologram plate comprising anoptically addressed SLM, and reproduce the three-dimensional image ofthe object. In this case, the three-dimensional image information of theobject can be recorded via interference of the input beam and thereference beam. The three-dimensional image information of the objectrecorded on the hologram plate can be reproduced into athree-dimensional image by the diffraction pattern generated by thereading beam. The erasing beam can be used to quickly remove the formeddiffraction pattern. On the other hand, the hologram plate can be movedbetween a position at which a three-dimensional image is inputted and aposition at which a three-dimensional image is reproduced.

ADVANTAGEOUS EFFECTS

According to the present disclosure, there can be provided a hologrammedium which can prevent light loss due to reflection, scattering, andabsorption caused by the influence of a light-transmissive substrate,can realize a higher refractive index modulation value and diffractionefficiency even in the range of a thin thickness, and can providelighter and smaller-sized products by removing the substrate whenapplied to VR (Virtual Reality) devices or AR (Augmented Reality)devices, and an optical element comprising the hologram medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a line profile of the shape of the surface of the hologrammedium of Example 1 measured using an atomic force microscope (AFM).

FIG. 2 is a line profile of the shape of the surface of the hologrammedium of Example 2 measured using an atomic force microscope (AFM).

FIG. 3 is a line profile of the shape of the surface of the hologrammedium of Example 3 measured using an atomic force microscope (AFM).

FIG. 4 is a line profile of the shape of the surface of the hologrammedium of Example 4 measured using an atomic force microscope (AFM).

FIG. 5 is a line profile of the shape of the surface of the hologrammedium of Comparative Example 1 measured using an atomic forcemicroscope (AFM).

FIG. 6 is a line profile of the shape of the surface of the hologrammedium of Comparative Example 2 measured using an atomic forcemicroscope (AFM).

FIG. 7 is a line profile of the shape of the surface of the hologrammedium of Comparative Example 3 measured using an atomic forcemicroscope (AFM).

Hereinafter, the present disclosure will be described in more detail byway of the following examples. However, these examples are given forillustrative purposes only and are not intended to limit the scope ofthe present disclosure thereto.

PREPARATION EXAMPLE Preparation Example 1: Preparation Method of a(meth)acrylate-based (co)polymer in which a silane-based FunctionalGroup is Located in a Branch Chain

154 g of butyl acrylate and 46 g of KBM-503(3-methacryloxypropyltrimethoxysilane) were added to a 2 L jacketreactor and diluted with 800 g of ethyl acetate. The reactiontemperature was set at about 60 to 70° C., and stirring was carried outfor about 30 minutes to 1 hour. 0.0:2 g of n-dodecyl mercaptan wasfurther added, and stirring was further carried out for about 30minutes. Subsequently, 0.06 g of AIBN as a polymerization initiator wasadded and the polymerization was carried out at the reaction temperaturefor 4 hours or more and maintained until the residual acrylate contentbecame less than 1%, thereby preparing a (meth)acrylate-based(co)polymer in which a silane-based functional group was located in abranch chain (weight average molecular weight of about 500,000 to600,000, —Si(OR)₃ equivalent weight of 1019 g/eq.).

Preparation Example 2: Preparation Method of a (meth)acrylate-based(co)polymer in which a silane-based Functional Group is Located in aBranch Chain

180 g of butyl acrylate and 120 g of KBM-503(3-methacryloxypropyltrimethoxysilane) were added to a 2 L jacketreactor and diluted with 700 g of ethyl acetate. The reactiontemperature was set at 60 to 70° C., and stirring was carried out forabout 30 minutes to 1 hour. 0.03 g of n-dodecyl mercaptan was furtheradded, and stirring was further carried out for about 30 minutes.Subsequently, 0.09 g of AIBN as a polymerization initiator was added andthe polymerization was carried out at the reaction temperature for 4hours or more and maintained until the residual acrylate content becameless than 1%, thereby preparing a (meth)acrylate-based (co)polymer inwhich a silane-based functional group was located in a branch chain(weight average molecular weight of about 500,000 to 600,000, —Si(OR)₃equivalent weight=586 g/eq.).

Preparation Example 3: Preparation Method of a (meth)acrylate-based(co)polymer in which a silane-based Functional Group is Located in aBranch Chain

255 g of butyl acrylate and 45 g of KBM-503(3-methacryloxypropyltrimethoxysilane) were added to a 2L jacket reactorand diluted with 700 g of ethyl acetate. The reaction temperature wasset at 60 to 70° C., and stirring was carried out for about 30 minutesto 1 hour. 0.03 g of n-dodecyl mercaptan was further added, and stirringwas further carried out for about 30 minutes. Subsequently, 0.09 g ofAIBN as a polymerization initiator was added and the polymerization wascarried out at the reaction temperature for 4 hours or more andmaintained until the residual acrylate content became less than 1%,thereby preparing a (meth)acrylate-based (co)polymer in which asilane-based functional group was located in a branch chain (weightaverage molecular weight of about 500,000 to 600,000, —Si(OR)₃equivalent weight of 1562 g/eq.).

Preparation Example 4: Preparation Method of Silane Crosslinking Agent

In a 1000 ml flask, 19.79 g of KBE-9007(3-isocyanatopropyltriethoxysilane), 12.80 g of PEG-400 and 0.57 g ofDBTDL were added and diluted with 300 g of tetrahydrofuran. The mixturewas stirred at room temperature until it was confirmed by TLC that allthe reactants were consumed, and then the reaction solvent wascompletely removed under reduced pressure.

28 g of a liquid product having a purity 95% or more was separated in ayield of 91% through column chromatography under a developing solutioncondition of dichloromethane:methyl alcohol=30:1, thereby obtaining theabove-mentioned silane crosslinking agent.

Preparation Example 5: Preparation Method of Non-Reactive Low RefractiveIndex Material

In a 1000 ml flask, 20.51 g of2,2′-((oxybis(1,1,2,2-tetrafluoroethane-2,1-diyl))bis(oxy))bis(2,2-difluoroethan-1-ol)was added and then dissolved in 500 g of tetrahydrofuran. 4.40 g ofsodium hydride (60% dispersion in mineral oil) was carefully added overseveral times while stirring at 0° C. After stirring at 0° C. for 20minutes, 12.50 ml of 2-methoxyethoxymethyl chloride was slowly dropped.When it was confirmed by ¹H NMR that all the reactants were consumed,the reaction solvent was completely removed under reduced pressure. Theorganic layer was collected by extracting with 300 g of dichloromethanethree times, and filtered with magnesium sulfate, and then the pressurewas reduced to remove all the dichloromethane to obtain 29 g of a liquidproduct having a purity of 95% or more in a yield of 98%.

Example: Preparation of Photopolymer Composition and Hologram Medium

1. Preparation of Photopolymer Composition

As shown in Table 1 or Table 2 below, the silane polymers obtained inPreparation Examples 1 to 3, photoreactive monomer (high refractiveindex acrylate, refractive index 1.600, HR 6022 [Miwon]), thenon-reactive low refractive index material of Preparation Example 5,tributyl phosphate ([TBP], molecular weight 266.31, refractive index1.424, manufactured by Sigma-Aldrich), Safranin O (dye, manufactured bySigma-Aldrich), Ebecryl P-415 (SK entis), Borate V (Spectra Group),Irgacure 250 (BASF), silicone-based reactive additive (Tego Rad 2500)and methyl isobutyl ketone (MIBK) were mixed in a state cutting off thelight, and stirred with a paste mixer for about 10 minutes to obtain atransparent coating solution.

The silane crosslinking agent obtained in Preparation Example 4 wasadded to the coating solution, and further stirred for about 10 minutes.Subsequently, 0.02 g of DBTDL as a catalyst was added to the coatingsolution, stirred for about 1 minute, and then coated in a thickness of6 μm onto a TAC substrate with a thickness of 80 μm using a Meyer barand dried at 40° C. for 1 hour.

Then, the sample was allowed to stand for 24 hours or more in a darkroom under constant temperature and humidity conditions of about 25° C.and 50% RH.

2. Preparation of Hologram Medium

(1) The above-prepared photopolymer-coated surfaces were laminated on aslide glass, and fixed so that a laser first passed through the glasssurface at the time of recording.

(2) Measurement of Diffraction Efficiency (η)

A holographic recording was done via interference of two interferencelights (reference light and object light), and the transmission-typerecording was done so that the two beams were incident on the same sideof the sample. The diffraction efficiencies are changed according to theincident angle of the two beams, and become non-slanted when theincident angles of the two beams are the same. In the non-slantedrecording, the diffraction grating is generated vertically to the filmbecause the incident angles of the two beams are the same on the normalbasis.

The recording (2θ=45°) was done in a transmission-type non-slantedmanner using a laser with a wavelength of 532 nm, and the diffractionefficiency (η) was calculated according to the following general formula1.

$\begin{matrix}{\eta = \frac{P_{D}}{P_{D} + P_{T}}} & \left\lbrack {{General}\mspace{14mu}{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

in the general formula 1, η is a diffraction efficiency, P_(D) is anoutput amount (mW/cm²) of the diffracted beam of a sample afterrecording, and P_(T) is an output amount (mW/cm²) of the transmittedbeam of the recorded sample.

(3) Measurement of Refractive Index Modulation Value (n)

The lossless dielectric grating of the transmission-type hologram cancalculate the refractive index modulation value (Δn) from the followinggeneral formula 2.

$\begin{matrix}{{\eta({DE})} = {{\sin^{2}\left( \sqrt{v^{2}} \right)} = {\sin^{2}\left( \frac{\pi\;\Delta\;{nd}}{\lambda\mspace{11mu}\cos\;\theta} \right)}}} & \left\lbrack {{General}\mspace{14mu}{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

in the general formula 2, d is a thickness of the photopolymer layer, Δnis a refractive index modulation value, η(DE) is a diffractionefficiency, and λ is a recording wavelength.

(4) Measurement of the Loss Amount of Laser (I_(loss))

The loss amount of laser (I_(loss)) can be calculated from the followinggeneral formula 3.

I _(loss)=1−{(P _(D) +P _(T))/I ₀}  [General Formula 3]

in the general formula 3, P_(D) is an output amount (mW/cm²) of thediffracted beam of the sample after recording, PT is an output amount(mW/cm²) of the transmitted beam of the recorded sample, and I₀ is anintensity of the recording light.

(5) Surface Observation of the Hologram Medium

The shape and size of the linear pattern formed on one surface of thehologram medium obtained in Examples were measured using an atomic forcemicroscope (XE7) manufactured by Park Systems. Then, the distancebetween the adjacent minimum and maximum values was measured through aline profile from the confirmed surface shape.

At this time, in the measurement result, the minimum value means a pointwhere the y-axis value decreases and then increases, the maximum valuemeans a point at which the y-axis value increases and then decreases,and “adjacent” means the most adjacent maximum value (or minimum value)with a height difference of 5 nm or more from the minimum value (ormaximum value).

As for the standard error of the adjacent minimum and maximum values,the distance between the adjacent minimum and maximum values having aheight difference of 5 nm or more was measured from the line profile ofthe surface profile measured by atomic force microscopy, and thestandard error was calculated according to the following general formula1.

SE=σ/√{square root over (n)}  [General Formula 1]

in the general formula 1, SE is a standard error, σ is a standarddeviation, and n is a number.

FIGS. 1 to 4 are line profiles of the shapes of the surfaces of thehologram media of Examples 1 to 4 measured using an atomic forcemicroscope.

(6) Measurement of Surface Energy

In the hologram medium obtained in Examples, the surface energy of onesurface making contact with the substrate was measured by the followingmethod.

For the sample, the substrate was cleanly peeled from the coating filmlaminated on the glass surface, and then the surface energy of onesurface making contact with the substrate was measured with a contactangle measuring device. The contact angle of di-water (Gebhardt) anddi-iodomethane (Owens) was measured as 10 points using a DSA100 contactangle measuring equipment form Kruss. After calculating the averagevalue, the surface energy was measured by converting the average contactangle into surface energy. In the measurement of the surface energy, thecontact angle was converted to the surface energy by using DropshapeAnalysis software and applying the following General Formula 2 of OWRK(Owen, Wendt, Rable, Kaelble) method.

γ_(L)(1+cos θ)=2√{square root over (γ_(S) ^(D)γ_(L) ^(D))}+2√{squareroot over (γ_(S) ^(P)γ_(L) ^(P))}  [General Formula 2]

TABLE 1 Measurement results of Experimental Examples of the photopolymercompositions (unit: g) of Examples and the hologram recording mediumprepared therefrom Category Example 1 Example 2 Example 3 Example 4(Meth) Preparation 23.1 23.1 acrylate- Example 1 based Preparation 19.3copolymer Example 2 Preparation 25.4 Example 3 Linear silane Preparation8.4 8.4 12.3 6.1 crosslinking Example 4 agent Photoreactive HR6022 31.531.5 31.5 31.5 monomer Dye safranin O 0.1 0.1 0.1 0.1 Amine EbecrylP-115 1.7 1.7 1.7 1.7 Borate salt Borate V 0.3 0.3 0.3 0.3 Onium saltIrgacure 250 0.1 0.1 0.1 0.1 Non-reactive Tributyl 0 17.2 17.2 17.2plasticizer phosphate (TBP) Non-reactive Preparation 34.4 17.2 17.2 17.2low refractive Example 5 material(P3) Catalyst DBTDL(dibutyltin 0.020.02 0.02 0.02 dilaurate) Additive Tego Rad 2500 0.3 0.3 0.3 0.3 SolventMIBK 300 300 300 300 Coating thickness (unit: μm) 6 6 6 6 I_(loss)(%) 2519 21 20 Surface energy 52 mN/m 53 mN/m 51 mN/m 55 mN/m Standard error(SE) 5.72 nm 2.26 nm 6.72 nm 4.30 nm Diffraction efficiency (η) 67% 69%80% 46% Δn 0.025 0.027 0.030 0.022 * Non-reactive plasticizer: Tributylphosphate (molecular weight of 2.66.31, refractive index of 1.424,purchased from Sigma-Aldrich)

Comparative Example: Preparation of Hologram Media

(1) Synthesis of Polyol

34.5 g of methyl acrylate, 57.5 g of butyl acrylate, and 8 g of4-hydroxy butyl acrylate were added to a 2 L jacket reactor, and dilutedwith 150 g of ethyl acetate. Stirring was performed for about 1 hourwhile maintaining the temperature of the jacket reactor at 60 to 70° C.Then, 0.035 g of n-dodecyl mercaptan was further added to the reactor,and further stirring was performed for about 30 minutes. Then, 0.04 g ofa polymerization initiator AIBN (2,2′-azo-bisisobutyronitrile) was addedthereto, and polymerized at a temperature of about 70° C. for about 4hours, and maintained until the content of the residual acrylate monomerbecame 1% by weight, and thereby the polyol was synthesized. At thistime, the obtained polyol had a weight average molecular weight of about700,000 in terms of polystyrene measured by GPC method, and the OHequivalent measured using KOH titration was 1802 g/OH mol.

(2) Preparation of Photopolymer Composition

The polyol of Preparation Example 1, the photoreactive monomer (highrefractive acrylate, refractive index of 1.600, HR6022 [Miwon]),safranin O (dye, manufactured by Sigma Aldrich), the non-reactive lowrefractive material of Preparation Example 5, (tributyl phosphate [TBP],molecular weight of 266.31, refractive index of 1.424, manufactured bySigma Aldrich), Ebecryl P-115 (SK entis), Borate V (Spectra group),Irgacure 250 (BASF), and methyl isobutyl ketone (MIBK) were mixed in astate cutting off the light, and stirred with a paste mixer for about 10minutes to obtain a transparent coating solution.

MFA-75X (Asahi Kasei, hexa-functional isocyanate, diluted to 75% byweight in xylene) was added to the coating solution, and further stirredfor about 5-10 minutes. DBTDL (dibutyltin dilaurate) as a catalyst wasadded thereto, stirred for about 1 minute, and then coated in athickness of 7 μm onto a TAC substrate with a thickness of 80 μm using aMeyer bar and dried at 40° C. for 1 hour.

(3) Preparation of Hologram Medium

A hologram medium was prepared in the same manner as in the aboveExamples, and the diffraction efficiency (η), the refractive indexmodulation value (n) and the laser loss (I_(loss)) were measuredaccording to the same method and the same conditions as in Examples.

(4) Surface Observation of Hologram Media.

The shape and size of the linear pattern formed on one surface of thehologram media obtained in Comparative Examples and the surface of thehologram media were measured and observed in the same manner as inExamples.

FIGS. 5 to 7 are line profiles of the shapes of the surfaces of thehologram media of Comparative Examples 1 to 4 measured using an atomicforce microscope.

TABLE 2 Comparative Comparative Comparative Category Example 1 Example 2Example 3 Polyol Preparation 26.2 26.2 26.2 Example 1 Isocyanate MFA-75X6.4 6.4 6.4 Photoreactive HR6022 31 31 31 monomer Dye safranin O 0.1 0.10.1 Amine Ebecryl P-115 1.7 1.7 1.7 Borate salt Borate V 0.3 0.3 0.3Onium salt Irgacure 250 0.1 0.1 0.1 Non-reactive Tributyl 16.9 33.8plasticizer(TBP) phosphate Non-reactive low Preparation 16.9 33.8refractive Example 5 material (P3) Catalyst DBTDL(dibutyltin 0.02 0.020.02 dilaurate) Additive Tego Rad 2500 0.5 0.5 0.5 Solvent MIBK 400 400400 Coating thickness (unit: μm) 7 7 7 Surface energy 40 mN/m 42 mN/m 38mN/m Standard error (SE) 187.98 nm 232.23 nm 150.50 nm Δn 0.010 0.0110.013 Diffraction efficiency (η) 19% 22% 28%

As seen from FIGS. 1 to 4, it was confirmed that in the hologram mediaof Examples, the standard error of the distance in the cross-sectionaldirection of the polymer substrate between the maximum value of theheight of the one fine protrusion and the minimum value of the height ofthe adjacent fine protrusion is 20 nm or less. As shown in Tables 1 and2, and that the hologram media of Examples have a refractive indexmodulation value (ΔN) of 0.020 or more and a diffraction efficiency of40% or more.

Further, as shown in Table 1 above, it was confirmed that the other onesurface of the hologram recording medium of Examples had a surfaceenergy of about 51 to 55 N/m, and it was also confirmed that it waseasily peeled off from a TAC substrate used in the manufacturing processof the hologram recording medium.

In contrast, as seen in FIGS. 5 to 7, it was confirmed that in thehologram media of Examples, the standard error of the distance in thecross-sectional direction of the polymer substrate between the maximumvalue of the height of the one fine protrusion and the minimum value ofthe height of the adjacent fine protrusion exceeds 20 nm, and in suchhologram media, image distortion may occur or additional scattering orinterference of diffracted light may occur, and diffraction efficiencymay also decrease. Specifically, as shown in Tables 1 and 2, it wasconfirmed that the photopolymer-coated films provided by thecompositions of Comparative Examples have a relatively low diffractionefficiency of 30% or less and a refractive index modulation value (Δn)of 0.015 or less as compared with Examples.

In addition, as shown in Table 2 above, it was confirmed that the otherone surface of the hologram recording medium of Comparative Examples hada surface energy of about 38 to 40 N/m. However, these hologramrecording media did not peel off from the TAC substrate used in themanufacturing process, and therefore, there is a limit in actualapplications in that the optical design must be proceeded inconsideration of the reflection, scattering, and absorption effects bythe substrate.

1. A hologram recording medium having one surface with a higher surfaceenergy than a polymer resin layer of a base substrate containing atleast one polymer selected from the group consisting of triacetylcellulose, alicyclic olefin polymer and polyethylene terephthalate. 2.The hologram recording medium according to claim 1, wherein a surfaceenergy of any one surface of the hologram recording medium is at least50 mN/m.
 3. The hologram recording medium according to claim 1, whereinthe one surface of the hologram recording medium has a surface energy of50 mN/m to 60 mN/m.
 4. The hologram recording medium according to claim1, wherein the hologram medium comprises a polymer substrate including apolymer resin having a silane-based functional group in a main chain ora branched chain of the polymer.
 5. The hologram recording mediumaccording to claim 4, wherein a fine pattern is formed on a surface ofthe polymer substrate that is the other surface of the hologramrecording medium.
 6. The hologram recording medium according to claim 5,wherein the fine pattern has a shape including two or more fineprotrusions in which a maximum value of the height, and a minimum valueof the height are alternately repeated based on the cross section of thepolymer substrate, and a standard error of the distance in thecross-sectional direction of the polymer substrate between the maximumvalue of the height of the one fine protrusion and the minimum value ofthe height of the adjacent fine protrusion is 20 nm less.
 7. Thehologram recording medium according to claim 6, wherein the distance inthe cross-sectional direction of the polymer substrate between themaximum value of the height of the one fine projection and the minimumvalue of the adjacent fine projection is 0.2 μm to 2 μm.
 8. The hologramrecording medium according to claim 6, wherein an amplitude which is adifference between the maximum value of the height of the one fineprojection and the minimum value of the height of the adjacent fineprojection is 5 to 50 nm.
 9. The hologram recording medium according toclaim 4, wherein the polymer substrate includes a cross-linked productbetween a polymer matrix including a (meth)acrylate-based (co)polymerhaving a silane-based functional group in a branched chain, and a silanecrosslinking agent; and a photoreactive monomer.
 10. The hologramrecording medium according to claim 9, wherein the polymer matrixincludes 10 parts by weight to 90 parts by weight of the silanecrosslinking agent based on 100 parts by weight of the(meth)acrylate-based (co)polymer.
 11. The hologram recording mediumaccording to claim 9, wherein the (meth)acrylate-based(co)polymer havinga silane-based functional group in a branched chain has an equivalentweight of the silane-based functional group of 300 g/eq. to 2000 g/eq.12. The hologram recording medium according to claim 9, wherein thesilane crosslinking agent includes a linear polyether main chain havinga weight average molecular weight of 100 to 2000 and a silane-basedfunctional group bonded to the terminal or branched chain of the mainchain.
 13. The hologram recording medium according to claim 4, whereinthe polymer substrate further includes a fluorine-based compoundincluding at least one functional group selected from the groupconsisting of an ether group, an ester group and an amide group, and atleast two difluoromethylene groups.
 14. An optical element comprisingthe hologram medium of claim
 1. 15. The hologram recording mediumaccording to claim 2, wherein the one surface of the hologram recordingmedium has a surface energy of 50 mN/m to 60 mN/m.
 16. The hologramrecording medium according to claim 2, wherein the hologram mediumcomprises a polymer substrate including a polymer resin having asilane-based functional group in a main chain or a branched chain of thepolymer.
 17. An optical element comprising the hologram medium of claim2.